The present invention relates to a rubber composition suitable for a hollow core of a pressureless tennis ball, and more particularly to the rubber composition which contains a specified polybutadiene having a high cis content as a rubber component.
In rubber compositions for producing a hollow core of tennis balls, natural rubber has been generally used as the rubber component. The reason is that natural rubber has a good processability and moreover the cured product is superior in strength.
Two types of tennis balls are known, one being pressurized tennis balls wherein the hollow core is pressurized by means of air or a certain kind of gas to about 0.6-1.2 kg/cm.sup.2 higher pressure than atmospheric pressure, and the other being pressureless tennis balls wherein the internal pressure of the hollow core is approximately equal to atmospheric pressure, for example, the excess pressure being 0 or up to 0.4 kg/cm.sup.2.
The pressurized tennis balls have a good rebound properties and a good feeling of striking balls by a racket, since the pressure of air or the gas in the core contributes to improvements in rebound properties of the balls and feeling of striking the balls. However, air or the gas of super-atmospheric pressure contained in the core gradually diffuses out through the core wall owing to a pressure difference between the inside and outside of the core and the internal pressure decreases in several months. Consequently, the rebound properties of the balls are reduced, thus the flight of the balls is reduced and the balls are no longer satisfactorily used. It is accordingly necessary for the pressurized tennis balls to be used within a certain specified term after the manufacture, or to be kept in pressurized containers prior to use for preventing the decrease of the internal pressure. However, these procedures are inconvenient and expensive.
On the other hand, the pressureless tennis balls have no problem of the lowering in ball properties resulting from the decrease of the pressure in the core as encountered in pressurized tennis balls. However, the rebound properties, feeling of striking and the like of the pressureless tennis balls must rely on the rebound properties of the rubber composition itself which constitutes the core, because there is no contribution of the pressure in the core to the rebound properties and striking feeling of the balls. Accordingly, it is attempted to improve these properties by making the core wall thick so as to approach the rebound properties and striking feeling of pressurized tennis balls.
However, the improvements in rebound properties and the feeling of striking of the balls by only the increase of the core thickness has a limit. Also, it is impossible to increase the core thickness over a certain level from the viewpoint of the weight of balls. For these reasons, in order to obtain the rebound properties and the feeling of striking which are close to those of pressurized tennis balls, the core rubber composition itself must be improved.
The improvement of the core rubber composition may be achieved by examining fillers to be incorporated therein. However, since the pressureless tennis balls are produced so that the core wall has an increased thickness as compared with pressurized tennis balls, the specific gravity of the core must be reduced by that proportion, and accordingly since the amount of the fillers to be added is restricted, it is difficult to improve the core rubber composition by only the fillers.
It is an object of the present invention to provide a rubber composition which is suitable for producing a hollow core of a pressureless tennis ball and which can provide a pressureless tennis ball having improved rebound properties and good feeling of striking.
The above and other objects of the present invention will become apparent from the description hereinafter.